Optical amplifying fiber and process of producing the same

Abstract

An optical amplifying fiber including a clad, a first core provided inside the clad and containing Ge, a second core provided inside the first core and containing Er and Al, and a third core provided inside the second core and containing Ge. The second core has a refractive index higher than that of the clad, and the first and third cores have refractive indexes each of which is higher than that of the second core. Since the third core having the high refractive index is provided at a central portion, it is possible to make smaller a mode field diameter and hence to improve a conversion efficiency of pumping light into signal light. Further, since the second core contains Al as an amplification band width increasing element, it is possible to sufficiently ensure a wide amplification band width.

What is claimed is: 
     
       1. An optical amplifying fiber comprising: a clad having a first refractive index;   a first core, provided inside said clad, containing a refractive index increasing element selected from the group consisting of Ge and Ti and having a second refractive index higher than the first refractive index;   a second core, provided inside said first core, containing Er and Al and having a third refractive index higher than the first refractive index and lower than the second refractive index, the concentration of Al being 4 wt % or more; and   a third core, provided inside said second core, containing a refractive index increasing element selected from the group consisting of Ge and Ti and having a fourth refractive index higher than the third refractive index, the third core having a diameter of 1 μm or less.   
     
     
       2. An optical amplifying fiber according to claim 1, wherein a mode field diameter is 4.5 μm or less. 
     
     
       3. An optical amplifying fiber according to claim 1, wherein a diameter of said second core is 2 μm or less. 
     
     
       4. An optical amplifying fiber according to claim 1, wherein the fourth refractive index of said third core is nearly equal to the second refractive index of said first core. 
     
     
       5. An optical amplifying fiber according to claim 1, further comprising: a fourth core, provided inside said third core, containing a rare earth element and an amplification band width widening element and having a fifth refractive index higher than the first refractive index and lower than the second refractive index; and   a fifth core, provided inside said fourth core, containing a refractive index increasing element and having a sixth refractive index higher than the third and fifth refractive indexes.   
     
     
       6. An optical amplifying fiber according to claim 5, wherein the fifth refractive index of said fourth core is nearly equal to the third refractive index of said second core, and the sixth refractive index of said fifth core is nearly equal to the fourth refractive index of said third core. 
     
     
       7. An optical amplifying fiber according to claim 5, wherein said amplification band width widening element is selected from a group consisting of Al, Zn, Sn, and La.